How do you build a human being?
Well, like anything else, you follow the instructions.
If you’re baking a cake, that’s a recipe.
Constructing a skyscraper - a blueprint.
But if you’re building a human, those instructions are DNA,
and while they might be a little long and a bit fiddly to read,
in terms of detail, they’re difficult to fault.
In this day and age most people are at least vaguely familiar with DNA
but it can be easy to forget that DNA is both a relatively recent discovery,
and an incredibly significant one.
A single molecule,
replicated within almost every one of the hundred trillion cells in your body,
which contains the complete source code for what makes you you,
is a pretty dumbfounding discovery.
And, perhaps inevitably, once DNA was discovered
it didn’t take scientists long to start thinking about ways to mess with it.
This idea is known as gene editing,
and it’s taken on various forms over the past half century or so,
but for the most part it’s been a fairly slow moving area of research.
That is, until recently.
In 2020, scientists Jennifer Doudna and Emmanuelle Charpentier
were awarded the Nobel Prize in Chemistry
for their work on a technology known as CRISPR-Cas9.
The potential impact of this discovery is hard to overstate,
but in order to understand why that is,
we need to first understand a few basic things about DNA.
DNA is made up of four ingredients, or ‘bases’,
each referred to by a different letter.
In the same way the alphabet can be rearranged to write the New York Times,
The Complete Works of Shakespeare, or a recipe for vegan chocolate brownies,
DNA’s four letters can be arranged and rearranged
in different orders and various combinations
to create a genetic blueprint for any living thing -
from bacteria to blue whales.
A single strand of human DNA contains about three billion of these letters -
that’s a document roughly a million pages long,
so you probably won’t be surprised to hear that occasionally a typo or two creeps in.
“Ok,” you might say, “it’s three billion letters though -
surely a couple out of order isn’t that big a deal.”
Well, sometimes it isn’t.
But in the same way that changing a single detail of a recipe,
or a single character in source code can have dramatic outcomes,
small errors in genetic instructions can have big consequences.
Down Syndrome, Cystic Fibrosis, Muscular Dystrophy - all genetic diseases
caused by relatively small ‘typos’ in million-page genetic documents.
But what if we had a ‘find and replace’ function,
where you could hunt through those three billion letters,
track down an exact error, and replace it with a corrected version?
Game-changer, right?
Well, that is what CRISPR does.
CRISPR stands for
Clustered Regularly Interspaced Short Palindromic Repeats,
and it refers to a DNA pattern found in bacteria
which serves as a sort of 'bacterial immune system'.
When attacked by a virus,
the bacteria uses enzymes to cut out a slice of the virus’s DNA,
and then stores a copy of that sequence within its own DNA -
think of it like a bar putting up a picture of an unwelcome patron
so they’ll be easy to recognise in future.
If, later, the same virus does attack,
the bacteria uses the stored slice as a sort of targeting system,
directing DNA-cutting enzymes
- known as CAS -
to search for matching DNA, and when found, snip right through it.
What Doudna and Charpentier discovered
was a way to control both this targeting system and the DNA-cutting enzymes,
which makes CRISPR an easy-to-use pair of ‘genetic scissors’
capable of targeting and cutting
any specific sequence within this impossibly long document.
The really cool part though is that
once a cut has been made, DNA takes care of everything else.
You see, DNA is great at repairing itself,
and so will patch itself up using any nearby DNA with matching broken ends.
So if you can snip out the problematic typo,
and then just leave a piece of replacement DNA from a healthy donor ‘lying around’,
the DNA will grab it and do the rest of the hard work for you.
And just like that, gene editing has become a reality.
This discovery opens an almost unimaginable number of doors -
incredible results have already been seen in treating sickle-cell anaemia,
and the technology is finding uses in other fields, such as agriculture.
But it doesn’t take a vivid imagination
to see that some of these doors lead to pretty unnerving places.
Curing genetic diseases is one thing, but the ethics of made-to-order babies
or making changes which can be passed down to future generations
are a lot more complicated.
And while most of those applications are out of reach right now,
the fact that CRISPR is accelerating our understanding of DNA
means that they probably won’t be forever.
The ability to edit the blueprint for life
has dizzying potential, and we’ve only just begun to scratch the surface.
Like any powerful tool,
CRISPR has the capacity to be used for good or for bad,
and this is precisely why it’s important to understand it -
so that when the time comes, we can make informed decisions
about the ways it can be used to improve our lives,
and where we need to draw the line.